The field of accounting has been evolving for many centuries. The Egyptian pharaohs, Roman emperors and European monarchs all had scores of accountants working for them, keeping track of taxes, war booty and governmental distributions. While modern accountants use ledger books, computers and software instead of clay tablets or papyrus scrolls to record their financial data, the basic processes of accounting – such as calculating the bottom line and determining amounts due based on ownership percentage – have not really changed all that much.
TL;DR (Too Long; Didn't Read)
Double underlining in accounting is typically used to indicate a grand total.
The Bottom Line
The term "the bottom line" has been used in common parlance to refer to the final word on a subject since the late 1960s, but the term actually comes from the field of accounting. The bottom line in accounting is the double-underlined final figure at the bottom of a column indicating a total profit or loss. However, some creative accountant types started using the term in other contexts, and the analogy obviously resonated with the public as the term was picked up by the media and has become so frequently used that it is almost a cliche today.
Single Underlining in Accounting
Single underlining in accounting is generally used to indicate a subtotal. For example, in an annual financial statement, the sales for the first quarter would be single underlined, as that amount is going to be added to the sales for the second, third and fourth quarters to ultimately come up with an annual total for the bottom line. Similarly, a balance sheet showing categories of the company's assets and liabilities would use a single underline under the total for each category of assets or liabilities.
Double Underlining in Accounting
Double underlining in accounting is typically used to indicate a grand total. Double underlining only appears in the figure at the bottom of a column of a financial statement or the like, and indicates the completion of that specific accounting procedure. For example, a financial statement showing the company's sales for four quarters would use double underlining under the grand total for all quarters. Likely, a balance sheet would use double underlining both under the total assets and total liabilities amounts.
Bolding and Dollar Signs
Bolding is sometimes used instead of or in addition to double underlining to indicate a grand total. Bolding is sometimes used for subtotals and double underlining for grand totals. By accounting convention, the first entry in a column will have a dollar sign and the rest will not, but there is some variation on this practice. Accounting software like Microsoft Excel usually have ways to set preferences for formatting figures.